1. Field of the Invention
This invention relates data processing for geophysical exploration, more specifically related to building velocity models for interpreting and refining a subsurface model in geological exploration.
2. Description of the Related Art
One of the methods used by the industry for obtaining images of the subsurface geology is wave-equation migration. Wave-equation migration uses a “one-way” or “two-way” wave equation to propagate the recorded data (sometimes referred to as the “receiver” wavefield) back into the subsurface. Simultaneously, the propagation of the wave field due to the seismic source is modeled throughout the subsurface. This wavefield is commonly referred to as the source wavefield. Migration with the “one-way” equation is commonly referred to as wavefield-extrapolation migration (WEM), whereas migration with the “two-way” equation is referred to as reverse-time migration (RTM). At a number of finely regularly sampled locations throughout the subsurface, the propagated source and receiver wavefields are combined in a way that provides an image of the subsurface geology.
Reflection operators are related to common-image gathers (CIGs) that are the subject of an extensive literature. There are different types of CIG. For example, one type of CIG could be constructed from the energy at zero-time on the reflection operators, if the operators are computed for a sampled range of depths. Such a CIG would then be a panel of migrated energy displayed as a function of the depth z and x, where x is the lateral separation of the down-going and up-going wavefield components. The latter are conventionally sampled at constant midpoint.
The analysis of certain types of CIG produced during prestack depth migration is a standard means of refining a depth migration velocity model. Events on CIGs produced by prestack Kirchhoff migration are horizontal when the migration velocity is exact. The horizontal property can be used as a tool for testing the velocity field or for deriving corrections which are then used to improve the velocity field. Similar comments may be made with regard to angle-domain common image gathers produced during midpoint-offset wave-equation migration. Recently so-called “extended” image gathers have been discussed in public.
Some references regarding CIGs are:    Biondi, B. and Symes, W., W., 2004. Angle-domain common-image gathers for migration velocity analysis by wavefield-continuation imaging, Geophysics, 69, 1283-1298    Sava, P. C. and Fomel, S., 2003. Angle-domain common-image gathers by wavefield continuation methods, Geophysics, 68, 1065-1074.    Rickett, J. E. and Sava, P. C., 2002. Offset and angle-domain common-angle point gathers for shot-profile migration, Geophysics, 67, 883-889.    Sava, P. and Biondi, B., 2004. Wave-equation migration velocity analysis. I. Theory, Geophysical Prospecting, vol. 52, 2004,    Sava, P. and Vasconcelos, I., 2009. Efficient computation of extended images by wavefield-based migration, 79th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts.    Vasconcelos, I, Save, P. and Douma, H., 2009. Wave-equation extended images via image-domain interferometry, 79th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts.
It is desirable to have a better method and apparatus to refine the velocity models for geological exploration